The present invention relates to a telephone ringer circuit for producing a call signal.
Prior art telephone ringer circuits with a mechanical bell often have poor tone quality and occupy a significant amount of space inside the telephone housing. To overcome these drawbacks, an electronic oscillator is used to drive a speaker, or the like, to produce a call signal. Such systems require external electric power to be supplied to the oscillator from the telephone line, because no components in the telephone circuit itself provided power to drive the oscillator.
More specifically, when the telephone receiver is left unlifted, a DC voltage of 48 volts from the telephone line is provided to the ringer circuit. In a ringer circuit with an oscillator, if a DC current flows into the ringer circuit, a telephone exchange may cause a malfunction. In order to prevent this malfunction, the DC resistance of the ringer circuit has to be high so that only the incoming ringer signals having low frequency and low current are passed. Thus, prior art telephone circuits have not included ringer circuits powered by telephone line signals.